A variety of tube fittings that are useful in the field of chromatography are commercially available. Most of these fittings employ a ferrule system that is swaged onto the tube.
One known design for making what is referred to as a "low dead volume connection" is called a "slip-free connector". With this connector, the depth of tubing penetration into a receiving fitting, and the compression force on a polymeric sealing ferrule, can be independently adjusted through the use of a two-part nut. The design of this connector is described as part of a chromatographic cartridge in U.S. Pat. No. 5,582,723. Unfortunately, this design can be difficult to use if the back part of the nut is too tight prior to installation into a receiving fitting. When this occurs, the tubing bottoms out in the receiving fitting and prevents compression of the sealing ferrule. Continued tightening of the "slip-free connector" without first loosening the back part of the nut eventually causes damage to the tubing or the receiving fitting.
A problem with tubing connection fittings in general is interchangeability from manufacturer to manufacturer. Different manufacturers use different penetration depths from the end of the sealing ferrule to the bottom of the receiving fitting, and penetration depths can even vary among different products from the same manufacturer. Because these fittings are assembled with the ferrule permanently swaged onto the tubing, using an already assembled tube in a different fitting can cause problems. If the tube extends too far past the end of the swaged-on ferrule the tube will bottom out in the fitting, which can make it difficult to get an adequate seal, or can damage the fitting. Conversely, if the tube does not extend far enough beyond the swaged-on ferrule the space not filled by the tubing results in dead volume. Use of the "slip-free connector" described above addresses the problem with interchangeability of fittings from one manufacturer to another, but as described above it can be difficult to use.
It is generally difficult to obtain reliable seals on 1/32.DELTA. tubing, which is common in chromatography applications. This size of tubing is typically thin-walled and, as the ferrule is tightened, the tubing can be deformed to the point where a reliable seal cannot be achieved. Continued tightening only aggravates this problem, and eventually the tubing is crimped closed or the sealing nut breaks.
U.S. Pat. No. 4,669,756 describes the use of a spring-loaded fitting to controllably force the ends of soft plastic tubing together to make a seal. In this case the ferrules are attached to the tubing ends and then pressed together with a spring. This is claimed to work with soft tubing, where deformation of the tubing ends occurs as they form the seal. As a result, this device has limited capability to seal to high pressure.
In addition, U.S. Pat. No. 4,732,672 describes a means of quickly mounting high-pressure liquid chromatography columns by using spring tension to force tubing into the end fittings of the columns. This same spring tension produces force on the sealing ferrule as well. The device is large and cumbersome, however, and the force on the tubing is achieved by a tight bend through a channel to a perpendicular dimension. Thin-walled tubing would collapse under forces required to keep it from being pushed out of the end fitting during high-pressure use. Also, the fact that the same spring is used to provide the tube holding force and the ferrule compression force means there is no independent control of these parameters.
U.S. Pat. Nos. 4,991,883, 5,163,722, and 5,163,215 describe spring-loaded connection devices wherein controlled pressure is applied to the sealing ferrule. This provides a constant sealing force on the ferrule member but does not provide for automatic bottoming out of the tubing in the receiving fitting.
U.S. Pat. Nos. 5,188,730 and 5,525,303 describe multiple-part connection devices that provide for dead volume removal by adjustment of a tubing member. These are similar to the "slip-free connection" device described earlier, and they suffer from the same problems, namely, that the rear supporting threaded member can be prematurely tightened, causing the tubing to prematurely bottom in the receiving fitting. This causes unreliable sealing and potential damage to the tubing or the receiving fitting.
The failure of current connection fittings to provide a reliable means of sealing tubing in a receiving fitting while automatically removing dead volume shows the need for a fitting that provides a simple, convenient yet robust means of connecting tubing to a receiving fitting while automatically removing dead volume in a small easy to use package.